In recent years, in wireless communication systems, such as mobile phone systems (cellular systems), the next-generation wireless communication technology is being discussed in order to implement wireless communication at higher speed and with higher capacity, more sophisticated functions, and the like.
For example, in standardization of the 3rd Generation Partnership Project Long Term Evolution (3GPP LTE), device-to-device (D2D) communication has been studied in which neighboring communication terminals directly communicate with each other without a base station, in order to reduce the traffic volume between the base station and the communication terminals. In this study, it is assumed that the D2D communication is performed by using an uplink resource, that is, uplink spectrum in Frequency Division Duplex (FDD) or an uplink subframe in Time Division Duplex (TDD).
Furthermore, as a conventional technology for the D2D communication, a technology has been proposed in which one of communication terminals measures path loss between the communication terminals by using a synchronization signal transmitted from the other one of the communication terminals, and determines transmission power of a response signal to be transmitted to the other one of the communication terminals on the basis of the measured path loss.
Examples of related-art are described in Japanese Laid-open Patent Publication No. 2013-034165, in Japanese National Publication of International Patent Application No. 2012-507975, in Japanese National Publication of International Patent Application No. 2006-522533, and in Patent Literature 4: International Publication Pamphlet No. 2009/122778.
However, the synchronization signal is transmitted only before the D2D communication starts. Therefore, in the conventional technology as described above, after the D2D communication has started, it is difficult to control the transmission power in accordance with a change in the path loss between the communication terminals due to movement or the like of the communication terminals during the D2D communication.
Furthermore, not all of the communication terminals transmit synchronization signals. Therefore, in some cases, estimation of path loss using a synchronization signal may be difficult.
Meanwhile, if a synchronization signal is periodically transmitted during the D2D communication in order to control the transmission power in accordance with a change in the path loss, a new communication resource to transmit the synchronization signal is needed and overhead increases.